1. Technical Field
The invention relates generally to field effect transistors, and more specifically to a nitride-based heterostructure field effect transistor having a quaternary strain matching layer for controlling strain.
2. Related Art
To date, III-N field effect transistors (i.e., a field effect transistor made of elements from group III and nitrogen) such as high microwave power heterojunction field effect transistors (HFETs), metal-oxide HFETs (MOSHFETs), and metal-insulator HFETs (MISHFETs) use AlGaN/GaN heterojunctions with the two-dimensional electron gas channel formed at the AlGaN/GaN heterojunction interface. This AlGaN/GaN heterojunction design has several limitations.
For example, in the heterojunction interface, carrier confinement is achieved by a self-consistent triangular potential quantum well. However, carriers can readily spill over into the buffer layer or the barrier layer. Such spill over increases low frequency noise and decreases transconductance. Further, carriers that have spilled over get trapped, thereby causing slow transient processes and a Radio Frequency (RF)-current collapse. Still further, a large gate voltage swing can lead to a significant strain modulation in both the buffer layer and barrier layer. The strain modulation can further contribute to current collapse.
To address these problems, some have suggested an AlGaN/GaN(or InGaN)/AlGaN double heterostructure design. While these devices show improvement in the low temperature (i.e., about 200 Kelvin) mobility, no improvement has been demonstrated for the room temperature mobility. Building on this, others have proposed a double heterostructure AlGaN/InGaN/GaN field effect transistor (DHFET) fabricated on an insulating SiC substrate. The DHFETs have demonstrated output RF powers as high as 4.3 Watts/millimeter (W/mm) in continuous wave (CW) mode, 6.3 W/mm in a pulsed mode, with a gain compression as low as four decibels. However, these devices have a relatively high level of low frequency noise that may indicate that these devices will have problems in yield and reliability. These problems are due to a large lattice mismatch between GaN, AlN, and InN, resulting in strong piezoelectric effects that significantly impact electrical and optical properties of III-N heterojunction devices.
As a result, a need exists for a field effect transistor having an increased lifetime and reliability, while exhibiting a reduced level of noise.